1. Field of the Invention
This invention relates generally to polyvinyl alcohol alloys which have desirable physical characteristics including low gas permeability, low moisture absorptivity, and relatively low melting points.
2. Description of the Prior Art
Polyvinyl alcohol (PVA) has the lowest gas permeability of any polymer yet synthesized, and has found widespread application as a gas barrier material, especially as an oxygen barrier material. Commercial polyvinyl alcohols have several failings, however. Due at least in part to their high polarity, their melting points are close to their decomposition points and, thus, they are not melt extrudable. Most films are cast from water or use large percentages of a polyhydric alcohol or polyethylene oxide (with or without added water) to plasticize the melt so that extrusion can be effected. Prior PVA polymers absorb relatively large quantities of moisture. The presence of moisture in turn raises the oxygen permeability of the polymers drastically.
One approach known in the art to alleviate these difficulties has been to introduce hydrocarbon units, especially ethylene units, into the chain of the polymer. The resulting materials are known as ethylenevinyl alcohol (EVOH) copolymers. EVOH copolymers have been used very successfully in a number of commercial applications, but are quite expensive. In addition, close control of composition and elimination of homopolymer by-products are required in the production of EVOH copolymers. A broad distribution of ethylene results in poor water vapor transmission rate properties. The presence of homopolymer leads to a tendency to develop gel specks and/or burn spots during the extrusion process.